The hyphenation of high performance liquid chromatography (HPLC) and mass spectrometry (MS) enables the selective and sensitive determination of various groups of analytes, because it combines the advantages of an effective separation technique and a highly selective detection method. Due to increased robustness of the instrumentation, HPLC-MS has become a widely used analytical technique in research and routine analysis.
However, some problems remain which are mainly caused by the difficulty of coupling a separation taking place in liquid phase with a detection technique that relies on the formation of gas phase ions. Different designs of interfaces have been developed to overcome this obstacle. Currently, the most common interfaces are electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI). HPLC-MS measurements with ESI and APCI have been reported to show excellent results for the determination of ionic or polar analytes, because these either are already ionized or can easily be ionized under the comparably soft conditions used for both ESI and APCI. Ionization typically occurs by protonation or deprotonation, but coordination of the analyte with other ions may also be used. Analytes of lower polarity are less accessible to the ESI or APCI processes resulting in low ionization efficiencies and losses in sensitivity. The scope of HPLC-MS on polar analytes is, however, unfortunate considering that analytes of lower polarity are best suited for separation by reversed phase liquid chromatography.
To overcome this limitation, only few attempts for the efficient ionization of less polar analytes have been done. Cole et al. in their Analytical Chemistry article used the electrospray interface for the electrochemical oxidation (ionization) of metallocenes. Another research group connected an electrochemical cell to thermospray-MS to study the electrooxidation of N,N-dimethylaniline. Brajter-Toth et al. used a combination of an electrochemical cell and particle beam mass spectrometry in a report in Analytical Chemistry. Also, the coupling of electrochemistry and thermospray-MS was applied by Brajter-Toth et al. for oxidative studies on uric acid. Van Berkel and co-workers have reported in Analytical Chemistry the determination of alcohols in saw palmetto fruit extracts and of alcohols and phenols in the oils of cloves, lemon, rose and peppermint using electrospray as an electrochemical reactor following a derivatization step with ferrocene-based reagents. Another approach suggested by Van Berkel et al. was the online coupling of different electrochemical flow cells with ESI-MS, either floated at or decoupled from the electrospray high voltage. Although the coupling of an electrochemical flow cell with MS gave promising results, no attempts for using this system after HPLC separation have been done.
Because the derivatization of alcohols and phenols with ferrocene-based reagents, e.g., can easily be accomplished and the resulting products should be well suited for electrochemical oxidation as well as for reversed phase liquid chromatography, we propose a new HPLC-electrochemistry-MS technique that has utility, e.g., for the determination of ferrocene derivatives.